Microlens formed of negative photoresist

ABSTRACT

By forming a microlens of negative photoresist, economical microlens fabrication processes may be used which, in some embodiments, may achieve microlenses having good optical clarity and high thermal stability. In one embodiment, a positive photoresist may be used as a pattern mask to transfer a pattern to the negative photoresist. The microlenses may be formed by dry etching the positive photoresist which acts as a mask to transfer a pattern to the underlying negative photoresist. At the same time a scratch protection layer may be formed over regions not overlying optical sensors.

This is a divisional of prior application Ser. No. 09/709,873, filedNov. 10, 2000, now U.S. Pat. No. 6,509,140 which is a divisional ofapplication Ser. No. 09/280,434, filed March 29, 1999, now U.S. Pat. No.6,249,034.

BACKGROUND

This invention relates generally to microlens arrays for variousapplications including increasing the fill factor of photosensitivearrays.

Conventional imaging systems may include a light sensing element such asa charge coupled device (CCD) or a complementary metal oxidesemiconductor (CMOS) sensor. The sensor may include one or more metallayers and interconnects, interlayer dielectric (ILD), a passivationlayer such as a nitride layer, a color filter array (CFA), planarizationover the CFA and a microlens array over the CFA. Conventionally,microlens arrays are formed using positive photoresist materials.

Conventional microlens fabrication involves forming a nitridepassivation, and then opening a bond pad. A polyimide layer is used totransfer a pattern to the positive photoresist. The polyimide layer isremoved and a color filter array is formed. Thereafter, the device isplanarized. Microlenses are defined in positive photoresist and then anultraviolet (UV) bleaching step is used to improve the transmissivity oroptical clarity of the positive photoresist.

Conventional processes may produce striations. In conventionalprocesses, the bond pad opening is formed before CFA formation. Thetopographic variation due to the surface cavity in the bond pad areas isa direct source of streaking patterns in CFAs which may later causestreaking in the resulting images.

Photobleaching may be used with positive photoresist because of theyellowing that may occur during processing. In addition positivephotoresists are inherently not transparent, even after a hard bake.This is believed to be due at least in part to the presence of photoacidcompounds in positive photoresist. Thus, the positive photoresist basedmicrolenses are photobleached using a deep ultraviolet (UV) source. Evenwith bleaching, a yellowing problem may arise upon exposure to heat andhumidity.

With existing positive photoresist microlens formation processes, thebond pads may be left with residues because the bond pad opening isformed before the microlens is formed. Thus, a final bond pad areaopening may be covered with residuals preventing good contact to thebond pad.

The thermal stability of positive photoresists is also limited. Whenexposed to high temperatures, positive photoresist may change shape orlose its optical clarity. Thus, it is generally desirable to avoid hightemperatures with positive photoresist based microlens arrays. However,avoiding high temperatures prevents using the surface mount processwhere the silicon chip and microlens can be heated up to over 200degrees during the solder reflow.

Additionally, in conventional processes, the regions adjacent to themicrolenses, which are not situated over photosensitive elements, aresubject to scratching during packaging because the nitride passivationis completely exposed. This scratching may ruin the devices and be anundetected source for light contamination.

Thus, there is a continuing need for improved photosensitive deviceshaving improved microlens arrays.

SUMMARY

In accordance with one embodiment, a microlens may include a lightcollecting element. The element may be formed of negative photoresist.

DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the processing of the negative photoresist in accordancewith one embodiment of the present invention;

FIG. 2 is an enlarged cross-sectional view showing a microlens in thecourse of fabrication;

FIG. 3 is a cross-sectional view showing the microlens of FIG. 2 afterensuing processing;

FIG. 4 is a cross-sectional view of the microlens structure of FIG. 3after continued processing;

FIG. 5 is a cross-sectional view of the microlens structure of FIG. 4after still additional processing;

FIG. 6 is a cross-sectional view of the microlens of FIG. 5 aftercompletion; and

FIG. 7 is an enlarged cross-sectional view of a photosensitive deviceformed in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION

A microlens may be formed from a negative photoresist layer. In oneembodiment of the invention, shown in FIG. 1, a transfer layer such as apositive photoresist layer may be used to transfer a desired pattern tothe negative photoresist layer. The microlens processing may be improvedand simplified using negative photoresist. In addition, a scratchprotection coating may be formed over the non-diode array area. Themicrolenses formed of negative photoresist may have high thermalstability and transparency in some embodiments.

Referring to FIG. 1, in one embodiment of the invention, the process offorming a microlens of negative photoresist begins by using aconventional nitride passivation step as indicated in block 10. A colorfilter array (CFA) is then formed (block 12) over the nitride layer bydeposition and patterning steps. Referring to FIG. 2, the nitride layer24 may cover the bond pad 22. The CFA layer 26 may be deposited andpatterned over the nitride passivation 24.

Referring back to FIG. 1, the structure is then planarized (block 14)using negative photoresist. The negative photoresist may be the samenegative photoresist that is conventionally used as the CFA resist. TheCFA resist is typically an acrylic based negative photoresist. In someembodiments, the acrylic photoresist may be cross-linked for highthermal stability. For example, some negative photoresists can withstandtemperatures over 200° C. for one hour without significant degradationin transmissivity. Other negative photoresists may be utilized as well.The negative photoresist 28 may be deposited by spin coating. Thenegative photoresist is then cured using conventional UV processingtechniques. Referring to FIG. 3, after depositing the negativephotoresist 28, an opening 30 may be formed (block 16, FIG. 1) to thebond pad using reactive ion etching (RIE)

Returning now to FIG. 1, the pattern of the desired microlens array maybe defined in the positive photoresist transfer layer, as indicated inblock 18. Referring to FIG. 4, the positive photoresist may be spincoated on the cured negative photoresist layer and then patterned tocreate the desired array of microlenses as indicated at 32. The spincoated photoresist over the bond pad 22 is removed during patterning.

The desired microlens shape may then be transferred to the negativephotoresist planarization layer as indicated in FIG. 1 at block 20 andin FIG. 5. Namely, the positive photoresist in block form, shown at 32in FIG. 4, may be melted to form the oval shape indicated at 34 in FIG.5. The pattern defined in the positive photoresist 32 may then betransferred to the negative photoresist 28 through dry etching as shownin FIG. 6. If desired, the dry etching step may be modified to increaseor decrease the curvature of the resulting microlens 38. In addition,since the previous step is a dry etching step, for example, using plasmaor RIE, it may automatically clean the bond pad 22. This avoids leavingresidues that would contaminate the bond pads.

In accordance with one embodiment of the present invention, thecompleted photosensitive array 42 may include a microlens array 38 asshown in FIG. 7. The CFA 26 may be situated over a nitride passivationlayer 24. The metal layers 46 and 48 may be separated by the ILD 50. Aphotosensitive device 54, such as a photodiode, is situated under eachmicrolens 38.

A negative resist protective coating 40 is formed over the regions whichare not overlying the photosensitive devices 54. The coating 40 may beformed by the same steps that are used to form the microlens array 38.This provides a scratch prevention layer. Scratches in this region canadversely effect the performance of the photosensitive devices.

Use of the negative photoresist process may reduce or eliminates theneed for the polyimide layer. Thus, polyimide ashing removal may beeliminated in some embodiments.

The CFA striation problem is also reduced or eliminated in someembodiments of the present invention since the bond pad opening is notdone before CFA formation, in some embodiments. Photobleaching and theyellowing issue may be avoided since the negative photoresist does notrequire bleaching. The negative photoresist also has high thermalstability and can withstand heating to temperatures over 200° C. for onehour without significant degradation in optical properties.

The positive photoresist used for pattern transfer may be a relativelycommonplace photoresist as opposed to special formulations currentlyused as microlens resist. A suitable positive photoresist is AZ4620 fromShipley Company, Marlborough, Mass. The use of common positivephotoresists may result in a cost saving compared to photoresists usedin conventional microlens formation processes.

In some embodiments, a high stability, high transparency negativephotoresist may be based on an epoxy acrylate resin. Particularly, epoxyacrylates having the fluorene moiety are particularly desirable. Forexample, the negative photoresist known as V259PA available from NipponChemical Company has desirable characteristics. Its formula is asfollows:

A hydroxide may be reacted with anhydride (indicated as a rectangleabove) to form a copolyester. The anhydride block may also contain acarboxylic group. The base resin may be advantageous over common acrylicresists because the fluorene pendent groups impart a high transparencyto the microlens. The carboxylic groups may enhance the base resinsolubility in alkaline solutions while producing good wetting adhesionto substrates. The base resin also exhibits low volume shrinkage duringcuring as compared to aliphatic acrylate.

The epoxy acrylates may have high glass transition temperatures on theorder of 250° C. versus 180° C. for aliphatic acrylate systems. Theepoxy acrylate may exhibit 90 percent light transmittance for 400-800nm. after heating to 200° C. for one hour and then 280° C. for anotherhour.

While the present invention has been described with respect to a limitednumber of embodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations that fall within thetrue spirit and scope of the present invention.

What is claimed is:
 1. A microlens comprising a light collecting elementformed of negative photoresist including an epoxy acrylate containing afluorene moiety.